Iron is involved in the pathogenesis of free radical tissue injury following inflammation. Activated leukocytes thus produce superoxide radicals which undergo dismutation to hydrogen peroxide and oxygen, and simultaneously reduce iron in ferritin. Fe.sup.++, in turn, reacts with hydrogen peroxide, producing the destructive hydroxyl radical. The radical reactions may be summarized: ##STR1## The presence of free iron is thus essential for the tissue damage, and removal of the metal from the system by chelation should reduce the tissue injury.
Iron chelators, including deferoxamine (DEF), have shown some effectiveness in reducing tissue injury, such as myocardial ischemia-reperfbsion injury. DEF must be given parenterally, but its toxicity and rapid excretion limit its effectiveness. By covalently linking DEF with high molecular weight substances, such as hydroxyethylstarch, the circulation lifetime was increased and toxicity decreased (Hallaway, et al., Proc. Nat. Acad Sci. USA, 86:10108, 1989). Other iron chelators have been explored in an effort to reduce tissue injury from free radicals (Voest, et al., Ann. Intern. Med, 120:490-499, 1994).
Polyethylene glycol (PEG) and its monomethyl ether (MPEG) have also been found to reduce tissue injury, although the mechanism is obscure. PEG is an amphiphilic polymer H(OCH.sub.2 CH2).sub.n OH that consists of a mixture of homologs with a range of similar molecular weights. Thus, MPEG 350 possesses an average molecular weight 350, and consists of a mixture of homologs with n=4 to 9, median n=7. The low molecular weight polymers PEG 200-600 are absorbed through the gastrointestinal tract when ingested orally and excreted unchanged in the urine. PEG is absorbed along with water directly through the intestinal mucosal cell membrane.
PEG 200-600, being nontoxic and biologically inert, has often been used as a vehicle for administration of drugs insoluble in water. In several investigations, the PEG vehicle alone was empirically found to exhibit significant biological activity, leading to further studies of low molecular weight PEG. As examples, PEG 400, when given intraperitoneally (IP) either before or shortly after x-irradiation of mice, conferred significant protection against lethality and morbidity (Shaeffer and Schellenberg, Int. J. Radiat. Oncol. Biol. Phys., 10:2329, 1984; Shaeffer, et al., Radiat. Res., 107:125, 1986). PEG 300 IP was shown to reduce the CNS sequelae of experimental concussive brain injury (Clifton, et al., J. Neurotrama, 6:71, 1989).
PEG with a molecular weight around 400 is thus a uniquely nontoxic substance that exhibits a protective effect against injury to tissues. However, PEG with a molecular weight greater than 700 is not absorbed through the GI tract. The mechanism of the protective action of low molecular weight PEG has not been established, but probably involves interaction of PEG with the surface of lipid membranes or protein components. PEG aggregates near cell membranes, reduces water polarity at membrane surfaces, and increases hydrophobic interactions (Hoekstra, et al., J. Biol. Chem., 264:6786, 1989).
It is known that certain MPEG chelates can be effective iron chelators. For example, it is known that MPEG can be linked with iminodiacetate terminus (MIDA). ##STR2## wherein n=3 to 8.
Other chelates modified with MPEG include MPEG 550-deferoxamine (ferrioxamine), prepared by reacting MPEG molecular weight 550 with carbonyldiimidazole, followed by reaction of the resulting imidazolecarbonyl ester with deferoxamine base, forming a urethane linkage. The material was produced as a chelate for gadolinium, to be used as a renal magnetic resonance contrast agent (Duewell, et al., Invest. Radiol., 26:50, 1991). Deferoxamine is known to be an effective chelator for ferric iron.
MIDA can be prepared by converting MPEG 350 to the chloride by reaction with thionyl chloride according to Bueckmann et al., Biotechnology & Applied Biochem., 9:258-268, 1987, and to the iminodiacetate by reaction of the chloride with sodium iminodiacetate (Wuenschell et al., J. Chromatog. 543: 345-354, 1991). The methyl ester can be prepared with methanolic HCl. However, alternative MPEG chelates, which are more effective chelators and are non-toxic, have been sought after.